1. Field of the Invention
The present invention is directed to aqueously dispersed, heat-curable coating compositions containing a water-dispersible epoxy resin component and a water-dispersible blocked polyisocyanate component, to a process for producing coated substrates from these compositions and to the coated substrates produced therefrom.
2. Description of the Prior Art
Epoxy resins which are rendered water-dispersible by external and/or chemically incorporated emulsifiers have recently increased in importance due to the need to eliminate volatile organic solvents from these systems. Water-dispersible epoxy resin compositions containing non-ionic and cationic emulsifiers are disclosed in U.S. Pat. Nos. 4,048,179; 4,073,762; 4,304,700 and 4,315,044. The epoxy resin compositions also conventionally contain nitrogen-containing curing agents such as polyamines, polyamide resins or urea- or melamine-aldehyde resins. Even though these curing agents provide adequate film properties, some of them are potentially physiologically harmful while others must be shielded from the atmosphere or it becomes difficult to cure the compositions in which they are used into coatings.
It has surprisingly been discovered that the problems associated with these known coating compositions may be overcome by using water-dispersible blocked polyisocyanates as curing or cross-linking agents for epoxy resins made water-dispersible by external and/or chemically incorporated non-ionic emulsifiers. Water-dispersible blocked polyisocyanates are known as disclosed by U.S. Pat. Nos. 4,098,933 and 4,284,544; however, their primary utility has been as cross-linkers for aqueous polyurethane dispersions, polyester polyols or polyhydroxy polyacrylates.
It is an object of the present invention to provide aqueously dispersed coating compositions based on epoxy resins and blocked polyisocyanates which are stable in storage, unaffected by the atmosphere and cure under the influence of heat to form cross-linked coatings possessing improved impact resistance.